Local oscillator (LO) leakage is a well known and common problem in radio frequency (RF) receiver circuits. Ideally, a receiver should not transmit, or "leak", any energy at the LO frequency. However, higher frequency receivers employ correspondingly higher LO frequencies, at which it becomes very difficult to design and costly to manufacture receivers with acceptably low LO leakage.
Excessive LO leakage causes significant problems in many modern communications systems. One familiar example derives from the field of automotive radar detectors. Upon initial market introduction, low cost radar detectors proved quite effective in warning motorists of nearby transmissions from law enforcement officials. However, as the popularity of the inexpensive radar detectors rose, users experienced a pronounced increase in the frequency of false alarms. These false alarms, it turned out, were being triggered by the excessive LO leakage from other low cost automotive radar detectors. The eventual solution involved the introduction of more sophisticated and costly units which proved capable of discriminating between the LO leakage from motorists' detectors and the radar transmissions from law enforcement officials.
Generally speaking, however, such discrimination and elimination of LO leakage interference is not feasible since the frequency of the leaking LO signal can be exactly the same as that of the signal of interest. This is, for example, the reason why use of commercial FM broadcast receivers is typically prohibited on commercial airline flights. Under certain circumstances, the LO signal frequency from a commercial FM receiver may correspond identically to the air-to-ground communications frequency, thus posing a threat to the pilot's communications. The problem stems, in large part, from the high levels of LO leakage associated with most inexpensive, hand-held FM receivers.
LO frequencies are selected, at least in part, to mitigate potential interference with existing communications bands. In situations where the LO frequencies of a particular class of receivers fall within a known communications band, regulation of the maximum allowable LO leakage from such receivers is prudent. The Federal Communications Commission (FCC) routinely promulgates such regulations. For example, the maximum power that receivers may radiate in the 1-2 GHz range in the United States is -45 dBm.
LO leakage is particularly troublesome for receivers or frequency conversion circuits in which the LO band overlaps the RF input band. One obvious reason is that the overlap in LO and RF input frequencies may cause LO interference among densely spaced, similar type receivers. A more subtle reason derives from the fact that it is inherently more difficult to reduce LO leakage in a receiver or frequency conversion circuit in which the RF input and LO frequencies are relatively close. To appreciate this point, one must consider the typical causes of LO leakage.
FIG. 1 depicts the overall structure of the front end of a receiver or other frequency conversion device. RF amplifier 11 amplifies the RF input signal from antenna or cable input 10. Mixer 13 combines the amplified input signal 12 with the LO signal 14 generated by LO 16 to produce IF output signal 15. At high frequencies, the LO leakage of the simple mixer 13 is often undesirably high, due in large part to the parasitic capacitance of the devices available to implement mixer 13. Thus, node 12 often maintains a substantial signal at the LO frequency. To reduce the LO signal at the antenna input node 10, and thus the overall LO leakage, one must design RF input amplifier 11 to have a very low reverse gain at the LO frequency. More precisely, it is the ratio of the forward gain of RF amplifier 11 at the RF input frequency to the reverse gain of the same amplifier at the LO frequency which most strongly affects LO leakage.
Unfortunately, designing RF amplifier 11 to meet these specifications is neither easy nor cheap. First, as the frequency of the system increases, parasitic capacitances reduce the achievable ratio of forward to reverse gain in amplifier 11. To address this problem, RF amplifier 11 can be made frequency selective. For example, U.S. Pat. No. 4,662,001 shows a system that uses a frequency selective RF input amplifier. In this system, interstage tuned coupling circuit 24 (see FIG. 1 of the '001 patent) implements a tunable notch filter, designed to track the LO frequency and reduce the LO leakage in the system. Other known techniques employ phase cancellation of the LO signal at the input terminal; i.e., an appropriately phase shifted and scaled version of LO signal 14 is applied to RF input terminal 10 in order to compensate for, or "cancel", the LO leakage signal at input terminal 10. See, for example, U.S. Pat. Nos. 4,811,425 and 5,001,773. With most such phase cancellation techniques, the improvement attainable is at best an order of magnitude and more typically about a factor of two. Moreover, the hardware needed to implement the canceling feedback is non trivial.
Thus, there remains a significant need for a method and apparatus for reducing LO leakage in high frequency circuits.
There also exists a present need for a method and apparatus for reducing LO leakage in integrated receiver circuits.
Still further, there exists a need for a method and apparatus for reducing the effects of parasitic inductances in high frequency analog integrated circuits.
Yet another present need relates to a method and apparatus for reducing the LO leakage in integrated frequency conversion circuits caused by the parasitic inductances of integrated circuit packages.
A similar present need involves a method and apparatus for reducing LO leakage caused by the parasitic package inductances in thick film or other modularly encapsulated frequency conversion circuits or devices.
Still another need involves the reduction of inductive feedback or coupling through the power and/or ground terminals of encapsulated or integrated circuits and, in particular, a method and apparatus for reducing such coupling.
These and other objectives are satisfied, at least in part, by the present invention as set forth in broad terms immediately below, and further by means of a detailed description of the presently preferred embodiment.